Our invention concerns a press for sheet metal forming, and more specifically to a press of the double acting variety, having a die forced downwardly to engage a sheet metal blank between itself and a blank holder, and a punch thrusted upwardly to press the blank against the die. Still more specifically, our invention pertains to means in such a double acting press for adjustably varying the lowermost position of the punch with respect to the blank holder in order to allow the blank to be placed horizontally on the punch and the blank holder.
A double acting press has been known which comprises a die carried by an upper slide thereby to be forced downwardly to engage a sheet metal blank against a hollow blank holder, and a punch received in the blank holder and pushed upwardly by a lower slide to press the blank against the die. While the upper slide rigidly carries the die, the lower slide is not coupled thereto but moves into and out of abutting engagement therewith. Upon full descent of the lower slide, the punch comes to rest on ledges projecting interiorly from the hollow blank holder, allowing the lower slide to move out of engagement with the punch. The punch when resting on the blank holder ledges is in its normal, lowermost position, from which it is to be raised by the punch as the latter subsequently ascends into abutment against the punch.
In a double acting press of this type, the blank to be formed is placed horizontally on the blank holder while the punch is in its lowermost position. The punch when in its lowermost position must have its top flush with the top of the blank holder to hold the blank thereon in cooperation with the blank holder. Should the punch top be lower than the blank holder top, the blank might sag in the middle under its own weight, particularly when it is of large size. If the punch top were higher than the blank holder top, on the other hand, then the blank would not be neatly placed on the blank holder in the correct horizontal attitude. In either case, the formed part would suffer defects such as wrinkles and wavy edges.
Conventionally, the double acting press of this kind has had no built in means for the adjustment of the lowermost position of the punch with respect to the blank holder, the punch having been placed on the blank holder ledges when in its lowermost position. Common practice, then, has been to shim the punch on the blank holder ledges. We object to this conventional practice because of too much time and labor involved.